1. Field of the Invention
The present invention relates to a process and an apparatus for effectively removing high levels of silver in the presence of high levels of ferric/ferrous complexes in aqueous effluents, particularly spent photo finishing solutions.
2. Description of the Related Art
Over the past few decades, environmental concerns of the scientific community as well as the general public, have brought attention to the problem of heavy metal ions in the waste water, i.e., effluent, discharged by industrial operations. In response to this concern, industries which generated such metal ions as waste have devised numerous ways of treating their waste to recover the metal ions and recycle or dispose of them in an environmentally acceptable manner. In many cases the governments in the industrialized countries have improved environmental protection programs by passing laws setting limits on the amounts of toxic metal ions which may be discharged. Further, national and regional governmental agencies, e.g., the Environmental Protection Agency (EPA) in the United States, have been established to monitor and police the discharge of metal ions into the environment. While waste containing heavy metal ions being discharged into the environment has significantly decreased in recent years, there is a continuing need for more effective and efficient means of removing metal ions from waste water.
Often, the metal ions in industrial effluent are of sufficient commercial value that recovery and recycling of these metal ions is not only environmentally desirable, but economically profitable as well. For example, the photographic industry generates waste from product manufacturing and film processing containing a substantial amount of silver, both metallic and in the form of silver complexes. Federal regulations limit discharge of ionic silver into sewerage systems to less than 5 ppm. However, municipal standards in Europe are usually not more than 0.1 ppm. Further, silver historically has been, and will continue to be, a semi-precious metal and valuable industrial material. Therefore, the photographic industry has the dual incentive to recover silver from its effluent to comply with environmental laws and to recycle a valuable resource.
Conventional methods used to remove silver include metal ion displacement, sulfide precipitation, adsorption onto an ion-exchange resin, electrolysis, electrodialysis and reverse osmosis. These methods are not universally satisfactory and some are plagued with problems which restrict their use, such as secondary pollution. A particular method widely used in the art of silver recovery from an aqueous solution comprises reacting the solution with precipitating and flocculating agents to form solid precipitates containing the silver to be removed. The solid precipitates may be removed by well known methods in the art for liquid-solid separation, such as settling, decanting, centrifuging, and filtering. The liquid remaining after the precipitated silver has been removed may be further treated, if necessary, or disposed in the sewerage system in accordance with environmental regulations.
Regarding precipitation technology, many different non-polymeric sulfur compounds have been used as precipitants, such as sodium sulfide, dimethyl- or diethyl-dithiocarbamate salts (U.S. Pat. No. 5,205,939), and trimercapto-s-triazine (U.S. Pat. No. 5,288,728, U.S. Pat. No. 5,549,820), etc. However, since these non-polymeric precipitants tend to form extremely fine precipitates when reacted with silver, the settling and filtration of such fine precipitates becomes very difficult. Even with the assistance and complexity of adding high molecular-weight cationic amine flocculating agents, as disclosed in U.S. Pat. No. 5,205,939 and U.S. Pat. No. 5,437,792, the resulting discharge silver level after treatment reaches only down to 0.3-1.5 ppm of residual silver remaining in the treated waste.
A method for removing silver which uses water-soluble sulfur containing, hydrophilic, linear polymer chelating agents ("SPCA's"), from a solution containing silver halides and silver ions has been applied both as the sole chelating agent to remove silver from waste solutions of manufacturing photosensitive materials (U.S. Pat. No. 5,575,915) and of large-scale photo finishing plants (JP 6-65744 (application Ser. No. 08/399,658), U.S. Pat. No. 5,605,633, U.S. patent application Ser. No. 08/632,542), and in combination with various non-polymeric sulfur compounds, U.S. Pat. No. 4,670,160 and U.S. Pat. No. 4,731,187.
One advantage of using SPCA's is that the discharge silver level in the treated waste can consistently reach extremely low levels (e.g. &lt;0.04 ppm, or below the detection limit of standard atomic absorption spectroscopy) without added flocculating agent. However, the waste sources heretofore treated normally have initial low silver concentrations (e.g. 5-400 ppm) and usually contain insignificant amounts of ferric/ferrous complexes, normally present as the ferric/ferrous amino carboxylate complex in photographic effluent.
In addition, non-linear branched polymeric dithiocarbamate acid salts are reported as useful for precipitating metal contaminants from aqueous solution in U.S. Pat. No. 5,523,002. However, the data presented in this latter patent indicates that these salts are only marginally effective for removing silver to the desired low level.
U.S. Pat. No. 5,549,820 teaches an apparatus and a method for removing silver from photographic processing effluents. In general, the apparatus provides various means for the sequential addition of a non-polymeric sulfur precipitating agent followed by a flocculating agent. The flocculated precipitate is then separated from the liquid by settling tanks and/or filters. While the method and associated apparatus of the patent purports to be effective in removing silver over all typical effluent ranges of silver typically encountered in photographic processing waste, it does not disclose achieving the desired low levels of silver in the waste, i.e. less than 0.1 ppm, in the presence of high concentrations of ferric/ferrous complexes. Moreover, it does not use the SPCA's described in aforementioned U.S. Pat. No. 5,575,915; etc.
In recent years, small complete photo finishing labs, known as "minilabs" have appeared in photography shops, discount stores, and photo finishing kiosks in shopping centers. Typically a minilab is computerized and highly automated, requires only one operator, and allows a retailer to provide photo finishing service on site in one hour or less. The widespread use of minilabs has created a need for a highly efficient, cost effective means to treat the waste from minilabs so that waste can be disposed of in municipal sewerage systems.
The photographic minilab processing effluent solution contains extremely high levels of silver (typically in the range of 1.5-6 g/L). It also contains a significant amount of ferric/ferrous amino carboxylate complexes (typically in the range of 2-9 g/L as iron). The existence of such high levels of silver and ferric/ferrous complexes significantly reduces the effectiveness of the silver removing capability using SPCA's both as the sole chelating agent and also in combination with other non-polymeric sulfur compounds. The competition from the high ferric/ferrous complexes complex level in the solution renders it more difficult to reduce silver to very low levels, and consumes significant amounts of SPCA. In addition, the high initial silver concentration itself significantly reduces the silver-removing efficiency of the SPCA precipitant. Using a prior art treatment process (e.g. U.S. patent '915), the residual silver concentration in the treated minilab waste does not reach the similar low level as that of treated waste from a large-scale photo finishing plant, containing insignificant amounts of ferric/ferrous complexes, even at relatively much higher SPCA dosage.
With respect to the above mentioned SPCA's, branched varieties have been reported to be effective in removing specific metallic contaminants from aqueous solutions. For example, Carey (U.S. Pat. No. 5,523,002) describes the use of a water soluble, branched polydithiocarbamic acid salt for removing 75% or more of the following metal ions when dissolved in water: Cd, Cr, Co, Cu, Pb, Ni, Ti, V, and Zn. Other metal ions tested gave lower percentage reductions, and silver was only reduced by 30%, an amount that would not be considered efficient by current water treatment standards. In another example, Sparapany (U.S. Pat. No. 5,346,627) discloses the use of a water soluble, polyethyleneimine-dithiocarbamate (PEI-DTC) polymer to remove silver and ferric/ferrous complexes from waste water. However, the question of optimizing silver removal to very low levels in the presence of substantial quantities of ferric/ferrous complexes is not discussed.
There is therefore a need to 1) efficiently and cost effectively remove high quantities of silver, from aqueous effluents, particularly effluents from photo finishing processes which have high concentrations of ferric/ferrous complexes and other contaminants so as to attain very low residual levels of silver, i.e. under 0.1 ppm, and 2) apply branched PEI-DTC polymers for such economically selective and efficient silver removal.